In this paper,the complicated polymorphism of linagliptin was investigated along with conformational changes in the lattice.Three crystal structures including form A were obtained for the first time by the growth of single crystals and X-ray single-crystal diffraction analysis.The structure and molecular conformation of each crystal were analyzed in combination with two crystal structures reported in the literature.Two crystal forms,form A and form F,are found among the five crystals.Form F can exist as a variety of channel solvates,with lattice parameters varying slightly with the included solvent.There is a high energy barrier between the conformers in form A lattice and those in form F,which probably accounts for the difficult conversion from form F to form A.The molecular conformations are not identical in the varieties of form F,and their powder X-ray raditional diffraction(PXRD)patterns are also distinctly different.Finally,the conversion between form A and form F was revealed by common methods,such as suspension conversion and thermal analysis.
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